Variable pulley transmissions for transferring torque from an input or drive shaft to an output or driven shaft have been used for some time. In these transmissions, a first pulley constructed of a pair of flanges is mounted on the input shaft such that at least one of its flanges is axially movable with respect to its other flange. A second, similarly constructed and adjustable pulley is mounted on the output shaft. A flexible belt connects the two pulleys to transfer torque therebetween when the input shaft is driven. As the effective diameter of one pulley is changed and, simultaneously, the effective diameter of the other pulley is changed in the opposite direction, the drive ratio between the input and output shafts is adjusted in a smooth, continuous manner.
Automotive engineers have long recognized that the maximum operating efficiency of the engine could be achieved if the transmission could be controlled by adjusting to different loads and speed ratios, such that the engine is maintained and operated at its maximum efficiency operating conditions. This has not been possible when a conventional geared transmission is teamed with an engine. In the conventional geared transmission, the drive ratio is adjusted in discrete steps, rather than continuously. Accordingly, efforts have been directed to the use of a continuously variable transmission (CVT) of the type described above. These efforts have resulted in the production and marketing in Europe of the Daf passenger car, using a flexible, continuous rubber belt to drivingly interconnect the pulleys. Such a belt is subject to wear by reason of the torque it must handle and operates under severe temperature, vibration and other adverse conditions. To improve the belt life, efforts have been channeled to produce a flexible belt of metal, and some of these efforts are described in the patent literature.
Flexible metal belts for use with CVTs are generally of two varieties, those referred to as "push" belts and those referred to as "pull" belts. An example of a push belt is described in Van Doorne et al U.S. Pat. No. 3,720,113 and an example of a pull belt is described in Cole, Jr. et al U.S. Pat. No. 4,313,730. The Van Doorne et al belt comprises an endless carrier constructed of a plurality of nested metal bands, and an endless array of load blocks longitudinally movable along the carrier. Each block has edge surfaces for frictionally engaging the pulley flanges of a pulley transmission to transmit torque between the pulleys. The pull belt of Cole, Jr. et al utilizes an endless chain as the carrier, the sets of links of which are pivotally interconnected by pivot means, shown as round pins. Generally trapezoidal (when viewed from the front) load blocks encircle the links; however the load blocks are constrained against longitudinal movement along the chain by the pivot means.
The push belt as described is relatively expensive to manufacture because the nested carrier bands are precisely matched to each other. Such a belt must be installed and/or replaced as a complete, endless loop, and thus disassembly of parts of the pulley transmission is required, not only for the initial assembly, but also for replacement due to failure of one or more load blocks or one or more of its carrier bands.
The pull belt offers a less expensive alternative to the push belt. No precise matching of carrier parts is required. The belt can be assembled with a finite length, positioned around the pulleys, and the ends then connected by a pivot member. Thus disassembly of the pulleys is not required either for initial installation or for replacement of a belt.
Theoretically a load block, either on a push belt or a pull belt, entering a pulley is tangentially oriented with respect to the pulley. That is, the length dimension of the block is perpendicular to a radial line extending outwardly from the center of the pulley. Actually, the block may tilt and enter the pulley at some other angle. When the block's "window" or "windows", i.e., the opening or openings in which the carrier is located, are made with square defining edges, the tilting of the blocks cause the top and/or the bottom edges to dig into and damage the carrier, seriously affecting the carrier's ability to transmit torque. This weakens the carrier and leads to premature failure of the carrier. One attempt (not yet proven) to solving this problem is to make the top and bottom window defining surfaces slightly round or arcuate. This adds to the manufacturing costs of the belt.
Load blocks, during their torque transmitting operation, are pulled downwardly toward the center of each pulley and are thus subjected to transversely applied compressive loads which unduly stress the blocks and which can lead to their failure. The compressive stresses on load blocks can cause the lower, strut-like portion, to sag, twist or otherwise be distorted which can lead to failure of the block. One attempt to eliminate this result is to construct load blocks with multiple windows and to divide each set of links into sub-sets, the windows being separated by a generally centrally disposed load block column, such a column maintains the generally flat strut in its proper and desirable undistorted form. In the multiple window block construction, the number of links of a given thickness for a given width belt is determined by the window sizes. The multiple window block construction has proved successful in reducing the number of load block failures due to distortion; however, because the number of links in each set determines the ultimate tensile strength of the assembly, there is a limitation in the tensile strength of a given width belt.